Recent studies in murine models show that there is differential regulation and activation of B cell subsets, B1, marginal zone and follicular. Similarly, different molecular interactions govern the activation of naive and memory B cells. Thus, it is critically important to determine the nature of the B cells producing pathogenic anti-DNA antibodies in SLE in order to design therapeutic interventions that will effectively block their activation. We propose to use flow cytometry and single cell molecular genetic analysis to understand the perturbations in B cell phenotype and subset distributions to occur in patients with SLE. We will employ a novel methodology consisting of staining cells with fluorescent tetramers of a peptide mimetope for DNA to identify and isolate DNA reactive B cells. We hypothesize that we find subtle changes in global B cell profile in SLE patients, but we will find an exaggerated dysregulation present in the autoreactive B cells. We will identify patterns of dysregulation and will specifically address: 1) the B cell subset from which anti-DNA antibodies arise; 2) whether anti-DNA B cells have germ-line encoded autoreactivity or acquired autoreactivity by somatic mutation; 3) whether flares of disease activity are characterized by recruitment of new clones to the anti-DNA response or the activation of memory cells. These studies will help form the basis for rational interventions in SLE.